Protocol synchronization for HARQ

ABSTRACT

A method and apparatus according to the present invention addresses and/or prevents lost protocol synchronization in HARQ systems caused by ACK/NACK errors. One embodiment detects lost synchronization errors for NDI-based retransmission protocols and restores synchronization by sending an explicit RESET message. In response to the RESET message, the transmitter aborts the transmission of a current PDU and transmits a new PDU and corresponding NDI. Another embodiment prevents protocol synchronization errors by sending scheduling grants on a packet by packet basis. The receiver sends a subsequent explicit scheduling grant to the transmitter based on an error evaluation of a received PDU. The transmitter will not send the next PDU unless it receives the subsequent explicit scheduling grant.

This application is a continuation of U.S. application Ser. No.16/022,832, filed on Jun. 29, 2018, which is a continuation of U.S.application Ser. No. 15/267,773, filed on Sep. 16, 2016, granted as U.S.Pat. No. 10,033,522 on Jul. 24, 2018, which is a continuation of U.S.application Ser. No. 13/920,467, filed on Jun. 18, 2013, granted as U.S.Pat. No. 9,485,059 on Nov. 1, 2016, which is a continuation of U.S.application Ser. No. 12/444,915, filed Apr. 9, 2009, granted as U.S.Pat. No. 8,489,951 on Jul. 16, 2013, which was the National Stage ofInternational Application No. PCT/SE2007/050721, filed Oct. 9, 2007,which claims the benefit of Swedish Application No. 0602182-8, filedOct. 9, 2006, the disclosure of which is incorporated herein byreference.

BACKGROUND

The present invention relates to automatic retransmission protocols, andmore particularly to improved link layer protocol synchronizationassociated with automatic retransmission protocols.

The purpose of a wireless communication system is to successfullytransmit information from a transmitter to a receiver over acommunication channel. In wireless communication systems, bit errorsoccur during transmission due to noise and multi-path fading. A varietyof error control techniques are available for combating transmissionerrors and reducing bit errors. The hybrid automatic repeat request(HARQ) protocol, which combines ARQ with forward error correction (FEC),represents one exemplary error control technique. ARQ adds redundantbits or check bits to a protocol data unit (PDU) to enable detection oferrors at the receiver. If the receiver detects errors in the receivedPDU, the receiver may send a feedback message, (e.g., a NACK) on acontrol channel that request a repeat transmission of the PDU. FEC useserror-correcting codes to combat errors by adding redundancy to the PDUbefore it is transmitted. The added redundancy enables the receiver todetect and correct most errors that occur during transmission.

While HARQ provides robustness against link adaptation errors forhigh-speed downlink packet access (HSDPA) channels and enhanced uplinkchannels, problems may occur when the feedback message is erroneouslyinterpreted by the transmitter. Such interpretation errors may lead toresidual errors after HARQ operations. Further, such interpretationerrors may lead to the loss of link layer protocol synchronizationbetween the transmitter and receiver. For example, if the transmittermisinterprets a NACK associated with a PDU as an ACK, the transmitterwill transmit a new protocol data unit (PDU) instead of retransmittingthe previous PDU. Because the receiver does not receive the expectedretransmission, the receiver loses link layer protocol synchronizationwith the transmitter. Further, combining errors may occur at thereceiver when the receiver soft combines different PDUs, and attempts todecode the soft combination. It is therefore desirable to reduce theeffects of feedback message errors.

SUMMARY

One embodiment of the present invention detects lost synchronizationerrors for NDI-based retransmission protocols and restoressynchronization by sending an explicit RESET message. In response to theRESET message, the transmitter aborts the transmission of a current PDUand transmits a new PDU and corresponding NDI. The receiver may furtherdistinguish between recoverable and unrecoverable synchronizationerrors, and limit transmission of the RESET message to situations wherethe protocol synchronization error is unrecoverable.

Another embodiment of the present invention prevents protocolsynchronization errors by sending subsequent explicit scheduling grantsfor each PDU. Each time the receiver successfully receives a PDU, thereceiver sends a subsequent explicit scheduling grant to the transmitterto explicitly authorize the transmitter to send the next PDU. Thetransmitter will not send the next PDU unless it receives the subsequentexplicit scheduling grant. In some embodiments, the subsequent explicitscheduling grant may include an indicator that indicates to thetransmitter whether the next transmission should comprise aretransmission of a current PDU or a transmission of a new PDU.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary wireless transmitter in communication with awireless receiver.

FIG. 2 shows a conventional packet data communication between thetransmitter and the receiver.

FIGS. 3A and 3B show process diagrams for one exemplary NDI-basedsynchronization process according to the present invention.

FIG. 4 shows a block diagram of an exemplary receive processor.

FIGS. 5A-5D show exemplary packet data communications between thetransmitter and the receiver for the NDI-based synchronization processof FIGS. 3A and 3B.

FIGS. 6A and 6B show process diagrams for one exemplary NDI-lesssynchronization process according to the present invention.

FIGS. 7A-7E show exemplary packet data communications between thetransmitter and the receiver for the NDI-less synchronization process ofFIGS. 6A and 6B.

DETAILED DESCRIPTION

The present invention is described herein in the context of atransmitter and a receiver in a wireless communication system thatrelies on retransmission protocols, such as HARQ protocols. Thetransmitter and/or receiver described herein may be disposed in a mobilestation, a base station, or other network entity. The wireless systemoperates according to a predefined communication protocol, including butnot limited to, UMTS, GSM, etc.

FIG. 1 illustrates an exemplary transmitter 10 for transmitting protocoldata units (PDUs) to a receiver 20. The various blocks in FIG. 1represent functions of the transmit processor and receive processor.Some functions not material to the present invention have been omittedfor the sake of clarity. Thus, other functions may be present inaddition to those illustrated in FIG. 1 . The illustrated functions canbe implemented in one or more microprocessors, microcontrollers,hardware, or a combination thereof. The following describes the variousblocks in terms of the transmit processor 12. However, it will beappreciated that the same functions may be implemented by the receiveprocessor 22.

Transmit processor 12 includes a Packet Data Convergence Protocol (PDCP)module 13, a Radio Link Control (RLC) module 14, a Medium Access Control(MAC) module 15, a Physical Layer (PL) module 16, and a MAC scheduler17. Data to be transmitted in the form of IP packets enters the PDCPmodule 13 where the IP headers may be compressed to reduce the number ofbits transmitted over the air interface. The PDCP module 13 alsoperforms ciphering and deciphering of the IP packets for security. TheRLC module 14 ensures almost error free, in sequence delivery of packetsto higher layers, which is needed for certain types of communication.The RLC module 14 performs segmentation/concatenation, and handlesretransmission of erroneously received packets. The IP packets from thePDCP module 13 are used to create RLC PDUs, which may comprise divided(segmented) IP packets, concatenated IP packets, or a single IP packet.The MAC module 15 offers services to the RLC module 14 in the form oflogical channels. The MAC module 15 maps data received on variouslogical channels from the RLC module 14 to corresponding transportchannels. The MAC scheduler 17 is responsible for uplink and downlinkscheduling, which typically occurs at the base station. According to thepresent invention, the MAC scheduler 17 also receives feedback from theHybrid ARQ (HARQ) protocol process, as described in more detail below.Transport blocks are fed by the MAC module 15 to the PL module 16. ThePL module 16 handles coding/decoding, modulation/demodulation,interleaving, and spreading prior to transmission of one or more PDUs.As used herein, a PDU represents a block of data including both a bodyand a header. The PDU comprises a MAC PDU, and may comprise a singleblock of data, a segmented block of data, or a multiple concatenatedblocks of data.

WCDMA and LTE systems employ HARQ in the MAC module 15, 25 to handleerrors that occur during transmission. In broad terms, the HARQ protocolenables the receiver 20 to request retransmission of erroneouslyreceived PDUs. According to the HARQ protocol, the receive processor 22sends an HARQ feedback message on a control channel to the transmitter10 to indicate whether a current PDU was successfully received by thereceiver 20. For example, when the receiver 20 successfully receives thePDU, the receive processor 22 sends an ACK message to the transmitter10. In response to the ACK message, the transmit processor 12 transmitsthe succeeding PDU. When the receiver 20 does not successfully receivethe PDU, the receive processor 22 sends a NACK message to thetransmitter 10 to request that the transmit processor 12 retransmit thecurrent PDU. As discussed above, when the ACK or NACK feedback messageis misinterpreted at the transmitter 10, the subsequently transmittedPDU may differ from that expected by the receiver 20, which leads tolink layer protocol synchronization errors.

One conventional wireless system attempts to address this problem usinga new data indicator (NDI). The NDI is associated with a PDU, but isgenerally transmitted separately from the PDU on an uplink controlchannel. For 3GPP LTE, the transmitter 10 may transmit the NDI on aphysical downlink control channel (PDCCH). This enables the receiver tointerpret the NDI even when the receiver cannot decode the received PDU.The NDI may comprise a single bit that toggles each time a new PDU istransmitted from the transmitter to the receiver. Alternatively, the NDImay comprise multiple bits, where the same predetermined value is usedeach time the transmitter transmits a new PDU. It will be appreciatedthat the following description applies to both single-bit and multiplebit NDIs.

FIG. 2 illustrates one example of a packet data communication between atransmitter and a receiver that uses a one-bit NDI to facilitateprotocol synchronization. After the transmitter sends a buffer statusreport to the receiver, the receiver sends a scheduling grant to thetransmitter (A) on a downlink control channel, such as the PDCCH. Thescheduling grant comprises a multiple bit message with error protection,such as CRC, that identifies the transmitter, any retransmissionprotocols, such as HARQ, and the allocated wireless resources fortransmitting the PDUs, such as modulation, data rate, etc. In responseto the scheduling grant, the transmitter transmits a PDU with sequencenumber (SN)=1, redundancy version (RV) 1, and NDI=0. At (B), thereceiver successfully decodes the received PDU, and sends an ACK to thetransmitter (C). Subsequently, the transmitter transmits a second PDUwith RV1, SN=2, and NDI=1 (D). At (D), the receiver cannot decode thesecond PDU, and sends a NACK to the transmitter (E). In response, thetransmitter retransmits the SN=2 PDU with RV2 and NDI=1. However, duringtransmission, the NDI value toggles to NDI=0. While the receiverrecognizes that the NDI value does not match the expected NDI value, thereceiver assumes the new NDI value was caused by an ACK/NACK error, andtherefore, assumes the received PDU is a third PDU instead of aretransmission of the second PDU. Thus, the link layer protocolsynchronization between the receiver and transmitter has been lost.

One embodiment of the present invention addresses this problem bydetecting loss of synchronization and restoring synchronization bysending an explicit RESET message to the transmitter 10. The RESETmessage may comprise a 1-bit message that is CRC protected and sent on adownlink channel, such as the PDCCH, as part of a MAC control unit. Inresponse to receiving the RESET message, the transmitter abortstransmission of a current PDU and transmits a new PDU and correspondingNDI. Because the receiver 20 expects a new PDU and new NDI after sendingthe RESET message to the transmitter 10, the RESET message successfullyrestores protocol synchronization between the transmitter 10 and thereceiver 20.

FIGS. 3A and 3B show exemplary synchronization correction processes 100and 150 for a receiver 20 and transmitter 10, respectively, for aretransmission protocol according to the present invention. As shown inFIG. 3A, the receiver 20 receives a first PDU and a corresponding NDI(block 110). The receive processor 22 evaluates the received NDI todetermine if a protocol synchronization error has occurred. When aprotocol synchronization error is detected (block 120), the receiver 20transmits a RESET message to the transmitter 10 (block 130). If noprotocol synchronization error is detected (block 120), the receiver 20transmits an ACK/NACK message to the transmitter 10 (block 140). Asshown in FIG. 3B, transmitter 10 transmits the first PDU with thepredetermined NDI (block 160). If the transmitter 10 receives a RESETmessage (block 170), the transmitter 10 abandons the first PDU andtransmits a second PDU with a new NDI (block 160). If the transmitter 10does not receive a RESET message (block 170), the transmitter 10implements conditional retransmission operations based on the receivedACK/NACK message (block 180).

A modified version of this embodiment distinguishes between recoverableand unrecoverable protocol synchronization errors, and limitstransmission of the RESET message to situations where the protocolsynchronization error comprises an unrecoverable error. To that end, thereceiver 20 first determines the type of protocol synchronization error.When the protocol synchronization error comprises a recoverable error,the receive processor 22 in the receiver 20 corrects the error at thereceiver 20 and continues with normal operations. When the protocolsynchronization error comprises an unrecoverable error, the receiveprocessor 22 sends the RESET message to the transmitter 10.

FIG. 4 shows one exemplary receive processor 22 comprising an error unit24 for determining the type of protocol synchronization error. The errorunit 24 processes a current PDU with a previously received PDU todetermine the type of protocol synchronization error. In one embodiment,the current and previously received PDUs comprise sequentially receivedPDUs. In one embodiment, error unit 24 comprises a soft combiner 26 thatsoft combines the current PDU with the previously received PDU. Based onthe received NDI, the expected NDI, and whether or not the soft combinedPDUs are decodable, the receive processor 22 determines whether theprotocol synchronization error is recoverable or unrecoverable. Table 1shows exemplary scenarios for determining the type of protocolsynchronization error.

TABLE 1 NDI NDI Soft Previously Currently Expected Combination ReceivedReceived NDI Result Error Type 0 0 1 Decodable Recoverable ACK/NACKError Not Recoverable Decodable NDI Error 0 1 0 Decodable RecoverableNDI Error Not Unrecoverable Decodable ACK/NACK Error

FIGS. 5A-5D illustrate examples of the various scenarios shown in Table1 for detecting and correcting recoverable and unrecoverable protocolsynchronization errors according to the present invention. In FIG. 5A,receiver 20 successfully receives PDU₁ and the corresponding NDI=0.However, the transmitter 10 incorrectly interprets the ACK sent by thereceiver 20 as a NACK. As such, PDU₂ transmitted by transmitter 10comprises a retransmitted version of PDU₁ with NDI=0. Because thereceiver 20 is expecting a new PDU, the receiver expects the NDI totoggle from 0 to 1. However, the receive processor 22 notes that thereceived NDI=0, and therefore, recognizes that either a recoverableACK/NACK or a recoverable NDI error has occurred. To determine the errortype, the error unit 24 soft combines PDU₁ and PDU₂ based on theexpected type of redundancy used to retransmit PDU₁ as PDU₂. Because theresulting soft combination is decodable, the receive processor 22determines that the protocol synchronization error comprises arecoverable ACK/NACK error. To restore protocol synchronization, thereceive processor 22 retransmits an ACK to the transmitter 10. Thereceiver 20 subsequently processes PDU₁, PDU₂, or some combinationthereof according to conventional techniques to recover thecorresponding data. While not required, it will be appreciated that thereceive processor 22 may alternatively transmit a RESET message to thetransmitter 10.

In FIG. 5B, receiver 20 does not successfully receive PDU₁, andtherefore, sends a NACK to the transmitter 10. The transmitter 10incorrectly interprets the NACK as an ACK, and transmits a new PDU(PDU₂) with NDI=1. Because the received NDI=1 differs from the expectedNDI=0, the receive processor 22 recognizes that either an unrecoverableACK/NACK or a recoverable NDI error has occurred. The error unit 24 softcombines PDU₁ and PDU₂. Because the resulting soft combination is notdecodable, the receive processor 22 determines that the protocolsynchronization error comprises an unrecoverable ACK/NACK error. Torestore protocol synchronization, the receive processor 22 transmits aRESET message to transmitter 10.

In FIG. 5C, receiver 20 successfully receives PDU₁ and the correspondingNDI=0, and the transmitter 10 correctly interprets the ACK sent by thereceiver 20. As such, the transmitter 10 transmits a new PDU (PDU₂) withNDI=1. The transmitted NDI toggles during transmission, causing thereceived NDI to equal 0. Because the receiver 20 is expecting a new PDU,the receiver expects the NDI to equal 1. However, the receive processor22 notes that the received NDI=0, and therefore, recognizes that eithera recoverable ACK/NACK or a recoverable NDI error has occurred. Theerror unit 24 soft combines PDU₁ and PDU₂. Because the resulting softcombination is not decodable, the receive processor 22 determines thatthe protocol synchronization error comprises a recoverable NDI error. Torestore protocol synchronization, the receive processor 22 resets theNDI value to 1 at the receiver 20, transmits an ACK to the transmitter10, and processes the received PDU₂ using conventional decodingtechniques.

In FIG. 5D, receiver 20 does not successfully receive PDU₁, andtherefore sends a NACK to the transmitter 10. In response, transmitter10 retransmits version 2 of PDU₁ (PDU₂) with NDI=0. The transmitted NDItoggles during transmission, causing the received NDI to equal 1.Because the received NDI=1 differs from the expected NDI=0, the receiveprocessor 22 recognizes that either an unrecoverable ACK/NACK or arecoverable NDI error has occurred. The error unit 24 soft combines PDU₁and PDU₂. Because the resulting soft combination is decodable, thereceive processor 22 determines that the protocol synchronization errorcomprises a recoverable NDI error. To restore protocol synchronization,the receive processor 22 resets the NDI value to 0 at the receiver 20,transmits an ACK to the transmitter 10, and processes the received PDU₁combined with PDU₂ using conventional decoding techniques.

The above illustrates how sequentially received PDUs and theircorresponding NDIs may be used to identify and correct a single protocolsynchronization error. For multiple protocol synchronization errors,e.g., an ACK/NACK error coupled with an NDI error, the error unit 24 mayprocess three or more received PDUs to identify each protocolsynchronization error.

Another embodiment of the present invention eliminates the NDI andprevents protocol synchronization errors using subsequent explicitscheduling grants. More particularly, when the transmitter 10 receives ascheduling grant, the transmitter 10 transmits only the one PDU. TheHARQ unit in the MAC module 15 evaluates the received PDU for errors andsends HARQ feedback information to the MAC scheduler 17. The MACscheduler 27 in the receive processor 22 sends a subsequent explicitscheduling grant to the transmitter 10 based on the error evaluation.For example, when the receiver 20 successfully receives a PDU, thereceiver 20 sends a subsequent explicit scheduling grant to thetransmitter 10 to authorize the transmitter 10 to transmit the next PDU.Because scheduling grants comprise multiple bit messages that includeerror protection, such as CRC, they are not as susceptible to errors assingle bit ACK messages. Thus, using subsequent explicit schedulinggrants for each PDU significantly reduces the protocol synchronizationerrors associated with misinterpreted ACK/NACK messages. Further, usingthe explicit scheduling grants eliminates data associated uplink controlsignaling, such as NDIs.

FIGS. 6A and 6B show exemplary HARQ processes 200 and 250 for thereceiver 20 and transmitter 10, respectively, according to thisembodiment. As shown in FIG. 6A, the receiver 20 schedules one MAC PDU(block 210) and sends the corresponding scheduling grant to thetransmitter 10 (block 220) as discussed above. The receiver 20 receivesthe PDU (block 230) responsive to the explicit scheduling grant. When anerror is detected in the received PDU (block 240), the receiver 20requests retransmission, e.g., transmits a NACK message to thetransmitter 10 (block 250). If no errors are detected (block 240), thereceiver 20 schedules another PDU (block 210) and sends a subsequentexplicit scheduling grant to the transmitter 10 (block 220) to authorizethe transmission of the next PDU. While not required, the receiver 20may also transmit an ACK message to the transmitter 10. However, an ACKmessage does not authorize transmission of the next PDU unless the ACKmessage is accompanied by the subsequent explicit scheduling grant. Asshown in FIG. 6B, the transmitter 10 transmits a PDU responsive to anexplicit scheduling grant (block 260). Before transmitting the next PDU,the transmitter 10 waits for an explicit scheduling grant. If thetransmitter 10 receives a subsequent explicit scheduling grant (block270), the transmitter 10 transmits the next PDU (block 260). If thetransmitter 10 does not receive the subsequent explicit scheduling grant(block 270), the transmitter 10 may implement conditional retransmissionoperations based on the received ACK/NACK message (block 280), asdiscussed further below.

FIGS. 7A-7D illustrate multiple scenarios for sending successive PDUsusing subsequent explicit scheduling grants. In FIG. 7A, transmitter 10transmits redundancy version 1 of a PDU responsive to an explicitscheduling grant. The receiver 20 successfully receives the PDU, andtherefore, sends the subsequent explicit scheduling grant and sends anACK message to the transmitter 10. After correctly interpreting theexplicit scheduling grant, transmitter 10 transmits version 1 of thenext PDU.

In FIG. 7B, transmitter 10 transmits redundancy version 1 of a PDUresponsive to an explicit scheduling grant. The receiver 20 successfullyreceives the PDU, and therefore, sends a subsequent explicit schedulinggrant and sends an ACK message to the transmitter 10. The transmitter 10incorrectly interprets the ACK sent by the receiver 20 as a NACK, andtherefore, the received HARQ feedback message is inconsistent with thereceived scheduling grant. In this case, the explicit scheduling grantoverrides the HARQ feedback message, and the transmitter 10 transmitsversion 1 of the next succeeding PDU. Because the scheduling grant hasbetter error protection than an ACK/NACK signal, the use of thescheduling grant to trigger the transmission of a succeeding PDU avoidsthe protocol synchronization problems associated with the misinterpretedACK message.

In FIG. 7C, transmitter 10 transmits redundancy version 1 of a PDUresponsive to an explicit scheduling grant. The receiver 20 does notsuccessfully receive the PDU, and therefore, sends a NACK message to thetransmitter 10. In response, the transmitter 10 retransmits redundancyversion 2 of the PDU. The receiver 20 successfully receives theretransmitted PDU, and therefore, sends a subsequent explicit schedulinggrant and an ACK message to the transmitter 10. The transmitter 10correctly interprets the subsequent explicit scheduling grant, andtherefore, transmits redundancy version 1 of the next PDU to thereceiver 20.

FIGS. 7D and 7E illustrate how the scheduling grant embodiment mayhandle an incorrectly interpreted NACK message. Transmitter 10 transmitsredundancy version 1 of a PDU responsive to an explicit schedulinggrant. The receiver 20 does not successfully receive the PDU, andtherefore, sends a NACK message to the transmitter 10. Transmitter 10incorrectly interprets the NACK message as an ACK message. However,because transmitter 10 did not receive a subsequent explicit schedulinggrant, the transmitter 10 does not send the next PDU. As shown in FIG.7D, when receiver 20 fails to receive the expected retransmission, thereceiver 20 may abort all processing operations associated with thecurrent PDU. Alternatively, the receiver 20 may send additional NACKmessages, as shown in FIG. 7E. If the receiver 20 fails to receive aretransmission after sending some predetermined number of NACK messages,the receiver 20 may abort all processing operations associated with thecurrent PDU.

In some embodiments, subsequent explicit scheduling grants may be usedto explicitly request retransmission of a current PDU. The subsequentexplicit scheduling grants may include an indicator that indicateswhether the transmitter 10 should transmit the next PDU or shouldretransmit a current PDU. In this embodiment, the transmitter 10 eithertransmits a new PDU or a retransmitted version of a current PDU based onthe value of the indicator, regardless of the presence of an ACK or aNACK. In one example, the subsequent explicit scheduling grants mayinclude a redundancy version (RV) indicator that indicates a request fora succeeding PDU transmission when RV=1, and indicates a request forretransmission when RV 2. Alternatively, the subsequent explicitscheduling grants may include a single-bit or multi-bit transmissionflag that indicates a request for the next PDU transmission when theflag is set to a first predetermined value, and indicates a request fora retransmission when the flag is set to a different predeterminedvalue. For example, the subsequent explicit scheduling grants mayinclude a New Data Flag (NDF), where NDF=1 indicates a request for thenext PDU transmission while NDF=0 indicates a request for aretransmission of a current PDU. It will be appreciated that theexemplary list of indicators discussed above is not exhaustive.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A method for Hybrid Automatic Repeat Request(HARQ) link layer protocol synchronization for an uplink transmissionbetween a user equipment and a radio network node in a wirelesscommunication system, the method comprising: transmitting, between theuser equipment and the radio network node, a first protocol data unit inresponse to receiving, by the user equipment, an explicit schedulinggrant, evaluating, by the radio network node, the transmitted firstprotocol data unit for errors, transmitting, between the radio networknode and the user equipment, a HARQ feedback message for the transmittedfirst protocol data unit based on the error evaluation and transmitting,between the radio network node and the user equipment, a subsequentscheduling grant, transmitting, between the user equipment and the radionetwork node, a subsequent second protocol data unit distinct from thefirst protocol data unit in response to a subsequent explicit schedulinggrant received, by the user equipment, in addition to the HARQ feedbackmessage, wherein the subsequent explicit scheduling grant indicates tothe user equipment to be authorized for transmitting the second protocoldata unit, and re-transmitting, between the user equipment and the radionetwork node, the first protocol data unit in absence of receiving, bythe user equipment, a subsequent explicit scheduling grant upon the userequipment interpreting the HARQ feedback message as a negativeacknowledgement for the transmitted first protocol data unit.
 2. Themethod according to claim 1, wherein the HARQ feedback message is apositive acknowledgment for the transmitted first protocol data unit. 3.The method according to claim 1, the method further comprising:transmitting, between the radio network node and the user equipment, afurther HARQ feedback message for the first protocol data unit,interpreting, by the user equipment, the further HARQ feedback messageas a positive acknowledgement for the transmitted first protocol dataunit, and waiting, by the user equipment, for the subsequent explicitscheduling grant before the user equipment transmitting the secondprotocol data unit.
 4. The method according to claim 1, whereinsuccessive protocol data units are transmitted between the radio networknode and the user equipment in response to the user equipment receivingsubsequent explicit scheduling grants.
 5. The method according to claim1, wherein each protocol data unit is transmitted between the userequipment and the radio network node one at a time in response to theuser equipment receiving a scheduling grant.
 6. The method according toclaim 1, wherein the step of transmitting a subsequent second protocoldata unit is performed upon the user equipment interpreting the HARQfeedback message as a negative acknowledgement for the transmitted firstprotocol data unit.
 7. The method according to claim 1, wherein thetransmitted first protocol data unit has a redundancy version of one,wherein the re-transmitted first protocol data unit has a redundancyversion of two, and/or wherein the transmitted second protocol data unithas a redundancy version of one.
 8. The method according to claim 1,wherein the method applies to one HARQ process.
 9. The method accordingto claim 1, wherein the subsequent explicit scheduling grant istransmitted when no errors are detected, by the radio network node, inthe transmitted first protocol data unit.
 10. The method according toclaim 1, the method further comprising: transmitting, between the radionetwork node and the user equipment, a further HARQ feedback message fora transmitted further protocol data unit based on an error evaluationperformed by the radio network node, wherein the further HARQ feedbackmessage is a negative acknowledgement for the transmitted furtherprotocol data unit; detecting, by the radio network node, that are-transmitted further protocol data unit is not received in response tothe transmitted further HARQ feedback message; and aborting, by theradio network node, processing operations for the transmitted furtherprotocol data unit.
 11. The method according to claim 1, the methodfurther comprising: transmitting, between the radio network node and theuser equipment, a plurality of subsequent further HARQ feedback messagesfor a transmitted further protocol data unit based on an errorevaluation performed by the radio network node, wherein each of thesubsequent further HARQ feedback messages is a negative acknowledgementfor the transmitted further protocol data unit; detecting, by the radionetwork node, that a re-transmitted further protocol data unit is notreceived in response to the transmitted further HARQ feedback messages;and aborting, by the radio network node, processing operations for thetransmitted further protocol data unit.